(1) Field of the Invention
At the present state-of-the-art, can lids are tested for leaks on a helium tester, such as that made by the Varian Company, by sampling a given number of lids per hours, say approximately 50, from each press. From this sampling, testing is done manually on the selected lids to statistically determine if there are leaks in the overall press run. This is a fairly unreliable technique, since the sample is necessarily small with respect to a normal manufacturing rate of about four lids per second on each station. Since there are conventionally two stations per press, each press outputs about 28,800 lids per hour. Often as a result of this poor sampling technique, many leaking can end elements are passed without being found. Some of these leaking end elements may be found once they are installed as the bottom of a three piece can by subsequent testing. Significantly, however, others are never found because they are used as lids and, as such, the lids cannot be tested after the product is put inside.
Unlike prior art sampling techniques, the present invention teaches a new technique for inspecting each of the can lids while still on the press upon which they are manufactured. The present invention teaches a method and an apparatus for inspecting the can lids as they lie directly within a conveyor belt which moves away from the dies of the press, towards chutes which convey the lids to the subsequent packing region.
2. Description of the Prior Art
In addition to the above description of can lid sampling, the use of the continuous testing techniques have been taught by prior patents. Specifically, Linderman, et al., U.S. Pat. No. 3,416,659, of common assignment with the present invention, illustrates a technique for testing assembled cans which are continuously conveyed upon a belt through a photosensitive testing station. In our prior patent, a constant light source is used to illuminate the outside of a finished can assembly, and as such represents the previously referred to technique of determining leakage in a can end element once it is assembled as the bottom part of a semi-finished can. While this prior patent teaches using a light source emmitting light energy in the visible spectrum, the light source is maintained on as a can is passed through a testing zone. Unlike the present invention, this prior patent employs a modulated light source with a bandpass filter in order to screen out spurious signals. According to the present invention, there is no filtering element for discriminating between frequencies of impinging light, rather by providing a series of repetitive light impingements onto the can lid, the need for light filtering and band-pass filter devices is obviated, as will be explained hereinafter. Another prior art patent disclosing a system for detecting openings in opaque objects is the patent to Trimble et al., U.S. Pat. No. 3,826,923. Trimble employs a radiation source of extremely high intensity, further having a wave length of about 2,000 to 4,000 Angstroms. Trimble therefore employs extremely short wave length radiation for the purpose of penetrating sub-micron openings within an article to be tested, without penetrating through the walls of the article. It is significant that Trimble requires a radiation source in such a particular short wave length and further that the radiation source must be of extremely high intensities and not simply an intermittent radiation of light in the visible spectrum, as is taught herein. Categorically, the prior art represented by Trimble is not operable with a light source having a wave length in the visible spectrum, but rather requires a radiation admission of sufficiently short wave length to pass through an opening of about 300 millimicrons.
Unlike these representative prior art patents, the present invention may employ light in the visible frequency ranges, and depends upon a repetitive light impingement, together with an electronic circuitry for summing the responses of a photosensor system, so that the signal to noise ratio will be vastly improved. In neither of these representative prior art patents is there recognition that a repetitive flashing, for durations as short as 1/500 of a second, can be thereafter summed to improve the signal to noise ratio of the processed signal. Unlike these prior art testing techniques, the present invention recognizes that noise, which will normally be present in any photo-electric signal, is of a purely random nature. The present method takes advantages of the random nature of noise through a series of repetitive samplings which are then summed to significantly improve the signal to noise ratio. Because of the improvement obtained by the repetitive sampling technique, the use of significantly lower energy sources is possible.